Homopolymerization or copolymerization of vinyl chloride in the absence of solvents or dispersing agents

ABSTRACT

Process for the polymerization of vinyl chloride in the absence of solvents or dispersing agents in two stages. In the first stage the liquid monomers are polymerized to a conversion of from 1 to 15 percent; in the second stage polymerization is continued in the powder phase in the absence of liquid vinyl chloride. The process may be carried out continuously and gives very pure polymers which may be used for all purposes for which polyvinyl chloride is usually employed.

United States Patent Wisseroth et a1. Aug. 29, 1972 [54]HOMOPOLYMERIZATION OR [56] References Cited COPOLYMERIZATION OF VINYLUNITED STATES PATENTS CHLORIDE IN THE ABSENCE OF 3,522,227 7/1970 Thomas..260/92.8 SOLVENTS Q DISPERSING AGENTS 3,475,398 10/1969 Jobard..260/92.8 [72] Invent: g:;' w fi jg FOREIGN PATENTS OR APPLICATIONS 9bach; Peter Dodel, Ludwigshafen; 6,709,001 12/1967 Netherlands..260/92.8 Lothar Hoehr, Buerstadt; Rudolf Bel-beck, Frankemha]; RichardPrimary Examiner-Joseph L. Schofer Scholl, Gruenstadt, all of GermanyAssistant ExaminerJohn A. Donahue, Jr. Att0rneyJohnston, Root, OKeeffe,Keil, Thompson [73] Assignees Badische AnIlIn- & Soda-Fabnk Ak- & sh tlff tiengesellschaft Ludwigshafen(R- hine), Rhineland-Pfalz, Germany [57]ABSTRACT Filed: g- 19, 1970 Process for the polymerization of vinylchloride in the [21] Appl NO 65 236 absence of solvents or dispersingagents in two stages.

In the first stage the-liquid monomers are polymerized [30] Foreign'Application Priority Data to a conversion 0t from 1 to 15 percent; inthe second stage polymerization 18 continued In the powder phase Aug. 221969 Germany ..P 19 42 823.0 in the absence 0f liquid vinyl The processmay be carried out continuously and [52] US. Cl ..260/85.5 XA, 260/87.1,260/875 C, gives very pure polymers which may be used for a 260/87-726O/92-8 R purposes for which polyvinyl chloride is usually em- [51]Int. Cl. ..C08f 3/30, C08f 1/04, C08f 15/02 p10yed [58] Field of Search..260/85.5, 86.3, 87.5 C, 87.7,

10 Claims, No Drawings HOMOPOLYMERIZATION R COPOLYMERIZATION OF VINYLCHLORIDE IN THE ABSENCE OF SOLVENTS 0R DISPERSING AGENTS Thepolymerization of vinyl chloride in the absence of solvents ordispersing agents, referred to as bulk polymerization, begins in liquidvinyl chloride. Due to the insolubility of the polymer in the monomer,the reaction product separates out as a solid phase in the reactionmedium during polymerization. The polyvinyl chloride formed can absorbsufficient monomeric vinyl chloride to produce a slurry at conversionsas low as about l0-l 5 percent by weight and upwards, this slurryconsisting of agglomerates of solid polymer swollen by liquid monomer.At conversions above about 75-85 percent this slurry phase changes intoa powder phase which consists of dry polymer containing absorbed gaseousmonomer.

The polymerization of vinyl chloride is combined with strong heateffects so that the removal of the heat of polymerization is the mainproblem in bulk polymerization. The heat transfer in the highlyinhomogeneous slurry phase is particularly difficult; unless care istaken to ensure thorough mixing of the reaction mixture, overheatingreadily occurs and this may lead to discoloration and the formation oflumps. Moreover, the walls of the reaction vessel exhibit the growth ofincrustations which hinder the transfer of heat to the cooling jacket.

The prior art processes for the bulk polymerization of vinyl chloridemust therefore employ expensive mixing equipment such as tumbling millscontaining indifferent grinding media or complicated stirring systems inthe form of ribbon blades which are spirally arranged around a shaft andsweep the walls in order to improve heat transfer through them.

French Pat. No. 1,261,921 describes the use of this principle ofpolymerization in continuous operation. Apart from the fact thatcomplicated grinding equipment is necessary, it has been found that sucha process cannot be carried out continuously for long periods of timesince even when the said grinding equipment is used it is not possibleto completely avoid agglomeration in the slurry phase and consequentlythe polymerization reaction must be stopped at frequent intervals as aresult of the discharge equipment and pipes being clogged up.

A number of continuous processes for the polymerization of vinylchloride has already been described in which, at a conversion of aboutpercent, part of the polymer suspension is drawn off and subjected to aseparating operation to remove the liquid monomer. Since, however, suchseparating operations and the subsequent drying of the moist polymer arehighly expensive operations, such processes have not be adopted inindustry.

French Pat. No. l,382,072 describes a process for the bulkpolymerization of vinyl chloride in two stages in which fast stirring inthe first stage and slow stirring in the second stage lead to a highbulk density and uniform grain size distribution of the resultingproducts. This process is operated batchwise, however, and cannot beworked continuously due to the aforementioned difirculties which occurin the slurry phase in the second stage.

Finally, German Pat. No. 975,823 describes a method of polymerizingvinyl chloride in the powder phase in which liquid or gaseous vinylchloride is introduced into a bed of powdered polyvinyl chloride whereit is polymerized. In this way the formation of lumps in the slurryphase is avoided, but when attempts are made to operate this methodcontinuously it is found that, as the on-stream period increases, so toodoes the grain size of the resulting product and at the same time thegrain size distribution becomes less and less uniform.

It is therefore an object of the present invention to provide a processfor the manufacture of vinyl chloride polymers which may be operatedcontinuously for long periods of time without stoppages and which givesa product having satisfactory grain properties.

We have found that this object is achieved and that thehomopolymerization and copolymerization of vinyl chloride can be carriedout continuously in the absence of appreciable amounts of solvents ordispersing agents in two stages, each of which may comprise one or morepolymerization units connected in parallel or series, the conditions ofpolymerization being otherwise conventional, by polymerizing liquidmonomer in the first stage to a conversion of from 1 to 15 percent byweight, transferring the resulting suspension to the second stage wherethe monomer partial pressure is below the liquifying pressure at thegiven temperature, continuing polymerization in the powder phase andcontinuously removing the polymer from the last polymerization unit atthe rate at which it is formed.

The process is particularly suitable for the manufacture of polyvinylchloride. However, it may also be used for the production of copolymersof vinyl chloride with up to 30 percent by weight of comonomers such asvinylidene chloride, vinyl acetate, acrylonitrile, ethylene orpropylene.

The polymerization should be carried out in the absence of appreciablequantities of solvents or dispersing agents. This however does notexclude the possibility of negligible amounts of solvents being presentin the polymerization mixture which have for example been introduced asolvents for solid initiators. If their boiling point is sufficientlylow, they will be automatically removed from the reaction mixture whenthe finished polymer is discharged or the pressure released. Theirboiling point should preferably be below C, and their concentration withreference to the solid polymer should be below 1 percent by weight.

The temperature at which the polymerization is carried out is dependenton the desired molecular weight of the polymer to be produced, expressedby the K value (according to Fikentscher, Cellulosechernie, 13, 60(1932). It may be in the range 0 to 75 C. All initiators normally usedin the polymerization of vinyl chloride may be employed; the type ofinitiator depends on the desired polymerization temperature. If it isdesired to polymerize at relatively high temperatures, examples ofsuitable initiators are azodiisobutyronitrile, benzoyl peroxide andlauroyl peroxide. At somewhat lower temperatures it is advantageous touse isopropyl percarbonate, tertbutyl perpivalate or acetylcyclohexanesulfonyl peroxide. Mixtures of the said initiators may also be used.Polymerization may be carried out at very low temperatures if redoxsystems, such as combinations of acetylcyclohexane sulfonyl peroxidewith triethanolamine, ascorbic acid or acetaldehyde, are used. Thepolymerization may also be initiated at relatively low temperatures withaluminum alkylene or boron alkylene. It is possible to use a rapidlydecomposing initiator in the first stage and a slowly decomposinginitiator in the second stage or vice versa, or the polymerization maybe carried out with the same initiator in both stages, in which case nofurther initiator need be added in the second stage. Where redox systemsare used it is advantageous to add the reducing component in the secondstage.

Conventional additives may be present during the polymerization, such aspolymerization modifiers or hydrogen chloride binding agents.

1. The pre-polymerization in the first stage is carried out, in thesimplest case, in a vertical reactor in which agitation may be effectedby conventional stirrers, and if polymerization is only carried throughto low conversions at which the supension is still veryl liquid,agitation may be effected by boiling the suspension under reflux.Polymerization in this pre-reactor may be effected continuously bycontinuously adding liquid vinyl chloride and initiator and withdrawingthe resulting suspension at the same rate and transferring it to thesecond stage. The pre-polymerization may also be carried out batchwiseeither by stopping the reaction at the desired conversion level, forexample by cooling, or by adding only such an amount of initiator thatit is completely used up when the desired conversion has been reached.

In this case it is advantageous to employ two or more polymerizationsunits, pre-polymerization being carried out in one reactor whilst thesuspension is transferred from a second reactor to the second stage. Inprinciple, it is possible to use two or more polymerization unitsconnected in series in a cascade, polymerization being effected in thefirst reactor to a conversion of, say, percent and in a second reactorto a conversion of, say, percent.

The pre-polymerization carried out in the first stage is a bulkpolymerization reaction, i.e., a suspension of solid polymer is presentin liquid monomer. The polymerization temperature may be in the range 0to 75 C, preferably from 50 to 70 C. Since the system used is a closedsystem, the pressure is an equilibrium pressure and is thus directlydetermined by the polymerization temperature. Heat removal may beeffected by evaporative cooling or jacket cooling.

In the first stage, the polymerization is carried out to a conversion offrom I to percent by weight, i.e., the concentration of polymer in thesuspension should be between 1 and 15 percent by weight. Since thesuspension becomes somewhat viscous at conversions above 8 percent, itis preferred to operate at conversions below 8 percent. The best resultsare obtained when the conversion is between 4 and 6 percent.

2. The main polymerization carried out in the second stage may also beaffected in a vertical reactor. Since the powder can be moved veryeasily, a simple anchor agitator, for example, is sufficient to achievethorough mixing of the powder bed. However, it is advantageous to use anasymmetrical anchor agitator or a spiral agitator such as are describedin German Pat. No. 1,218,265 and German Pat. application No. P 15 57 042respectively. In this case, too, it is possible to connect a number ofpolymerization units in parallel so that a reactor in the first stagealternately supplies the suspension to a number of reactors in thesecond stage. It is however also possible to connect a number ofpolymerization units in series to form a cascade, powder beingcontinuously removed from a first reactor and passed to a second reactorwhere the polymerization is continued by forcing in gaseous vinylchloride, and so on. In this way a narrower particle size distributionmay be obtained. In this case it is possible to realize the idea of anendless cascade using a preferably horizontal reaction tube whichreceives the suspension at one end and from which the finished polymeris continuously withdrawn at the other end so that the pulverulentpolymerization mixture travels along the tube. If suitable stirringequipment is used to ensure that mixing takes place preferablytransversely to the direction of travel of the powder and only to aminor extent in the longitudinal direction of the tube, it is possibleto achieve very uniform growth of the polymer particles.

Polymer is removed from the last polymerization unit at the rate atwhich it is formed. This makes it possible to operate the second stage,the main polymerization stage, continuously. The removal of the polymermay be effected either via air looks or by means of dip tubes.

The polymerization temperature in the second stage may also be in therange 0 to C, preferably from 30 to 60 C. The polymerization reactionsin the two stages may, if desired, be carried out at differenttemperatures, the temperature in the first stage being preferably higherthan that in the second stage. The partial pressure of the monomer inthe second stage must be below the equilibrium pressure, i.e., it mustbe below the pressure which is required to liquify the monomer at thetemperature used. A pressure difference of at least 0.1 atmosphere isnecessary and it is advantageous to operate at a pressure difference ofat least 1 atmosphere.

The presence of inert gases, such as nitrogen or argon, enables thetotal pressure prevailing in the reaction vessels of the second stage tobe raised, provided that the partial pressure of the monomer remainsbelow the equilibrium pressure and the monomer is therefore notliquified.

If the polymerization temperature in the first stage is higher than inthe second stage, a pressure difference is created which is sufficientto transfer the suspension from the first stage to the second stage.During the transfer of the liquid phase which already contains suspendedparticles of polymer, it is advantageous to ensure that the solidpolymer which separates out on entering the second polymerization stageis distributed as uniformly as possible. This is usually achieved,however, by the atomizing effect which occurs as the suspension passesthe pressure gradient between the two polymerization stages, whicheffect may be improved by the use of nozzles as distribution aids.

The main advantage of the process of the invention over conventionalmethods of bulk polymerizing vinyl chloride is that the slurry phase isomitted so that polymerization in the second stage takes place in thedry powder phase, i.e., appreciable amounts of liquid vinyl chloride arenot present in the reactor. In order to keep the partial pressure of themonomer in the reactor of the second stage below the liquifyingpressure, it is necessary to remove the liquid vinyl chloride, which iscontained in the l-lS percent suspension transferred from the firststage, as quickly as possible from the reaction vessel in the secondstage. Basically, this may be effected in two ways:

a. If the pressure in the reaction vessel in the second stage is keptvery low and the suspension is slowly transferred from the first stagein small portions, the transferred vinyl chloride evaporatesimmediately. If the polymerization is caused to take place relativelyquickly by using an appropriate catalyst system or by selecting asuitable polymerization temperature, it is possible to remove the liquidvinyl chloride by evaporation and polymerization.

b. However, the simpler and technically more reliable method is to ventthe reaction vessel in the second stage for a short period immediatelyafter the suspension has been transferred from the first stage so thatthe pressure built up by evaporation of the vinyl chloride is loweredagain. In this way a portion of the liquid vinyl chloride, about 30 to90 percent by weight, preferably from 60 to 0 percent by weight, isremoved from the system and may be recycled to the first stage. The heatenergy required to evaporate the liquid vinyl chloride is taken from thepolymerization system in the second stage so that part of the heat ofpolymerization may be removed in this manner. The polymerization is notaffected if small amounts of liquid vinyl chloride are introduced intothe powder bed in the reactor of the second stage for short periods oftime during the transfer of the suspension from the first stage to thedry powder bed since these portions of liquid monomer immediatelyevaporate when the pressure is released and the formation of a slurry isavoided.

Consequently, the powder bed is dry in the second stage of thepolymerization in which the main part of the reaction takes place. Thispowder bed can be moved so easily that heat removal presents noproblems. In principle, it may be effected by conventional jacketcooling, but in industrial plant the removal of the heat of reaction isonly feasible when effected by the principle of internal cooling asdescribed, for example, in German Pat. No. 1,013,870, in which case theheat of reaction is substantially removed by evaporating monomer.

Owing to the good heat transfer and the omission of the slurry phase,the formation of agglomerates in the polymerization units in the secondstage is avoided with the result that large, very noisy grindingequipment is not necessary. For the first time the process of theinvention enables vinyl chloride to be polymerized continuously on acommercial scale.

It is no longer necessary to follow each polymerization operation byopening, cleaning and closing the reactors and initiating polymerizationanew.

The product obtained by the present method has properties which are notinferior to those of products obtained by conventional methods. Comparedwith suspension or emulsion polyvinyl chloride it has the advantage ofhigher purity because the addition of dispersing agents can be dispensedwith.

By varying the polymerization temperature any desired K value may beobtained.

The average diameter of the resulting polyvinyl chloride particles isgenerally in the range 50 to 200 p" If, for special applications such asthe manufacture of plasticized polyvinyl chloride, a very narrow grainsize distribution is necessary, this may be achieved in a simple mannerby effecting polymerization in the second stage in a horizontal reactiontube in which only vertical agitation is carried out. The product thusobtained then exhibits particularly good absorption of plasticizer.

EXAMPLE 1 A 10-1iter reactor provided with an anchor agitator containsabout 2.5 of liquid vinyl chloride which is kept at about 65 C byexternal cooling. The equilibrium pressure is about 11 atmospheres. Eachhour there is added 0.1 g of azodiisobutyronitrile through a small airlock in the cover of the reactor, the azodiisobutyronitrile beingflushed in with about 300 cm of liquid vinyl chloride. There is arrangeddownstream of this reactor a 25-liter reactor equipped with a spiralagitator and containing about 5 kg of pulverulent polyvinyl chloridekept under a monomer pressure of about 8 atmospheres and at atemperature of 65 C substantially by external cooling. About 250 em /hrof liquid vinyl chloride suspesnion containing not more than about 20 gof suspended solid polyvinyl chloride (conversion about 8 percent)obtained by polymerization in the first stage is transferred from thefirst reactor via a dip tube to the second stage as a result of thepressure difference between the two reactors. By simultaneously ventingthe second stage the pressure therein is kept at about 8 atmospheres.About 100-150 g/hr of dry pulverulent polyvinyl chloride is removed fromthe second stage through a dip tube and via a cyclone. The polymer was aK value of from about 65 to 70. The median of the grain sizedistribution is about 120-140 11., percent of the particles of polymerhaving a diameter between and 200 [.L. The polymerization was carriedout for several weeks continuously without any major trouble, nosubstantial change in the particle size being observed during thisperiod.

COMPARATIVE EXAMPLE 1a For comparison purposes, an attempt was made tooperate the process described in German Pat. No. 975,823 continuously.

Five kg of polyvinyl chloride was placed in a 25-liter reactor equippedwith an anchor agitator. The polymerization temperature was set at 60 C.At this temperature the equilibrium pressure of the monomeric vinylchloride was approximately 10 atmospheres. A solution of 0.1 g ofazodiisobutyronitrile in vinyl chloride per hour was added through anair lock. By briefly opening a valve connected to a vinyl chloride line,gaseous vinyl chloride was pressured into the reactor until the pressurehad reached about 8 atmospheres. After about 30 minutes the pressure haddropped to 7 atmospheres due to polymerization of the vinyl chloride,whereupon vinyl chloride was pressed in to'raise the pressure to 8atmospheres again. About g of polyvinyl chloride was discharged perhour. Initially, the polymer particles had an average diameter of aboutM; after the process had been operated for 1 week the average particlediameter had doubled, and

after 4 weeks the process had to be stopped because some of theparticles had become so large that the discharge lines had becomeblocked up.

COMPARATIVE EXAMPLE lb The procedure described in Example la wasfollowed except that liquid vinyl chloride was fed into the reactorinstead of gaseous vinyl chloride until the equilibrium pressure thereinhad reached 10 atmospheres. After the pressure had dropped to 9atmospheres (on the average after about 20 minutes) liquid vinylchloride was again pressed in to raise the pressure to 10 atmospheresagain. About 180 g of product was discharged per hour. The particlediameter doubled after an on-stream period of only 4 days.

EXAMPLE 2 The polymerization is carried out in the same reactors and onthe same basis as described in Example 1.

The polymerization initiator .used is acetylsulfonyl cyclohexaneperoxide in the form of a 10 percent solution in methylene chloride. Inthe first stage polymerization is carried out at 60 C under anequilibrium pressure of 9.5 atmospheres and with the addition of 0.05 gof initiator per hour. In the second stage the pressure is kept constantbetween 4.5 and 5 atmospheres at a temperature of 40 C. Since theinitiator is almost completely consumed in the first stage, freshinitiator must be added in the second stage, the amount being 0.25 g ofacetylsulfonyl cyclohexane peroxide per hour which is added via an airlock.

From 250 to 300 g of dry polyvinyl chloride is discharged per hour. TheK value of the product is between 66 and 69 and its bulk density is 0.3g/cm.

The median of the grain size distribution is about 150 [1,, 80 percentof the particles having a diameter between 90 and 250 ;1,. Owing to itsspecial grain structure the product has an outstanding absorptivecapacity for plasticizers.

EXAMPLE 3 The procedure of Example 2 is followed except that in thefirst stage polymerization is carried out at 40 C and a pressure of 7atmospheres with the addition of 0.25 g of acetylsulfonyl cyclohexaneperoxide (10 percent solution in methylene chloride) per hour. At thistemperature the initiator is only partly consumed in the first stage. Inthe second stage there is added, as reducing component 0.15 g oftriethanolamine (10 percent solution in methylene chloride) per hourthrough an air lock and the polymerization is carried out at 32 C and apressure of from 3.5 to 4 atmospheres.

From 150 to 200 g of a dry powder having a K value of from 68 to 72 anda bulk density of 0.3 g/cm is discharged per hour.

Over an on-strearn period of several weeks the median of the grain sizedistribution remains constant at about 130 11., 80 percent of theparticles having a diameter between 60 and 200 [.L.

EXAMPLE 4 The polymerization in the first stage is carried out as inExample 3 in a vertical stirred autoclave except that l g ofacetylsulfonyl cyclohexane peroxide is added per hour. The gas phasepolymerization in the second stage is carried out in this Example in ahorizontal tubular reactor having a ratio of length to diameter of 202iand equipped with a gate paddle agitator. In the second stagepolymerization is carried out at 40 C and a pressure of from 4.5 to 5atmospheres without the addition of further catalyst or accelerator.Here again, the pressure in the second stage is kept constant byreleasing the pressure for a short time after the polymer suspension hasbeen transferred from the first stage.

From 1.2 to 1.5 kg of product is discharged per hour. The product has aK value of from 65 to and a bulk density of 0.28 g/cm The median of theparticle size distribution is about 120 1.1., the grain sizedistribution being very narrow: 90 percent of the particles having adiameter between and 150 t.

We claim:

1. A continuous process for the manufacture of vinyl chloride polymersin the absence of appreciable amounts of solvents or dispersiong agentsin two stages, each of which may comprise one or more polymerizationunits connected in parallel or series, the polymerization being carriedout in the presence of free radical-forming initiators at temperaturesbetween 0 and 75C, the conditions of polymerization being otherwiseconventional, wherein in the first stage liquid monomer is polymerizedto a conversion of from 1 to 15 percent by weight, the resultingsuspension is transferred to the second stage where the monomer partialpressure is below the liquifying pressure at the given temperature, from30 to percent by weight of the liquid vinyl chloride is removed from thesystem by venting during said transfer or immediately after saidtransfer, from the suspension coming from the first stage by venting thesecond stage, polymerization is continued in the powder phase and thepolymer is continuously removed from the last polymerization unit at therate at which it is formed.

2. A process as claimed in claim 1 wherein vinyl chloride is polymerizedtogether with from 0 to 30 percent by weight of vinylidene chloride,vinyl acetate, acrylonitrile, ethylene or propylene.

3. A process as claimed in claim 1 wherein redox systems are used asinitiators.

4. A process as claimed in claim 1 wherein the initiators are suppliedto the polymerization vessel in the form of solutions inorganic solventshaving a boiling point below 75 C.

5. A process as claimed in claim 1 wherein the polymerizationtemperature is higher in the first stage than in the second stage.

6. A process as claimed in claim 1 wherein the polymerization is carriedout in the first stage at temperatures of from 50 to 70 C and in thesecond stage at temperatures of from 30 to 60 C.

7. A process as claimed in claim 1 wherein the polymerization in thefirst stage is carried out to a conversion of from 4 to 8 percent.

8. A process as claimed in claim 1 wherein the pressure in the firststage is at least 0.1 atmosphere higher than in the second stage.

9. A process as claimed in claim 1 wherein the pressure in the firststage is at least 1 atmosphere higher than in the second stage.

PC4050 UNITED STATES PATENT OFFICE 5 CERTIFICATE OF CORRECTION PatentNo. 5,687,917 Dated August 29, 1972 Inventor) Karl wisseroth et al It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 5, line 25, "60 to O percent"should read 60 to 80 percent Column6, line 14 "2.5 of" should read 2.5 L. of

Column 8, line 1 of Claim 6 "in Claim 1" should read in Claim Signed andsealed this 20th dey of March 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOT'ISCHALK Attesting Officer Commissionerof Patents

2. A process as claimed in claim 1 wherein vinyl chloride is polymerizedtogether with from 0 to 30 percent by weight of vinylidene chloride,vinyl acetate, acrylonitrile, ethylene or propylene.
 3. A process asclaimed in claim 1 wherein redox systems are used as initiators.
 4. Aprocess as claimed in claim 1 wherein the initiators are supplied to thepolymerization vessel in the form of solutions in organic solventshaving a boiling point below 75* C.
 5. A process as claimed in claim 1wherein the polymerization temperature is higher in the first stage thanin the second stage.
 6. A process as claimed in claim 1 wherein thepolymerization is carried out in the first stage at temPeratures of from50* to 70* C and in the second stage at temperatures of from 30* to 60*C.
 7. A process as claimed in claim 1 wherein the polymerization in thefirst stage is carried out to a conversion of from 4 to 8 percent.
 8. Aprocess as claimed in claim 1 wherein the pressure in the first stage isat least 0.1 atmosphere higher than in the second stage.
 9. A process asclaimed in claim 1 wherein the pressure in the first stage is at least 1atmosphere higher than in the second stage.
 10. A process as in claim 7wherein during the transfer of the suspension from the first stage tothe second stage from 60 to 80 percent by weight of the liquid vinylchloride is removed from the system by venting.